


In Hot Water

by Nanenna



Category: Undertale (Video Game)
Genre: Alternate Universe - Merpeople, Alternate Universe - Underfell (Undertale), Kustard (Undertale Ship), M/M, Mating Cycles/In Heat, Miscommunication, Oviposition, Patchwork AU, Sans/Underfell Sans (Undertale), Underfell Sans (Undertale), no editing we die like men
Language: English
Status: Completed
Published: 2020-04-12
Updated: 2020-04-12
Packaged: 2021-03-01 16:42:00
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,981
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23620249
Author URL: https://archiveofourown.org/users/Nanenna/pseuds/Nanenna
Summary: Sans has been confused ever since he tumbled into the ocean and found a thriving monster culture at the bottom, but today the confusion has reached record heights. For some reason Red keeps swimming around in front of him, all his fins spread wide and waving about, calling out to him to get his attention. If only Sans could figure out what he wanted or how to respond.
Relationships: Sans/Sans (Undertale)
Comments: 36
Kudos: 267





	In Hot Water

**Author's Note:**

  * Inspired by [Underfell](https://archiveofourown.org/external_works/763605) by Vic the Underfella. 



> Happy Easter! Enjoy some eggs and some good old idiots. I like them dumb, okay? >.>  
> Also there's some background SpicyHoney because I can't resist, ufu hu hu!
> 
> Now also available in Russian [в горячей воде](https://ficbook.net/readfic/9723387/25020220) A big thank you to Ravti for translating it!

Sans sat on the smooth surface of his new home on the ocean floor, sunlight weakly filtering through the water to faintly fall on his bones. It had been a few months since he tumbled into the depths and not bothered to pull himself back out for some reason (or for some _one_ , as Alphys liked to tease). He’d almost immediately given up clothes back then, they were heavy, uncomfortable, unwieldy, and what was the point when no one else bothered with fashion either? Sans was a go with the flow kind of guy, so long as the flow wasn’t a current that would drag him bodily from the comfortable sea city he lived in. Again.

Sans laced his phalanges behind his head and leaned back, eyes half lidded as he watched fish and merfolk swim by. Today was a lively day, it seemed like everyone in the whole city was out, flitting about in the sunlight that, while still weak compared to what Sans had grown used to on the Surface, had grown brighter and was out longer than when he’d first arrived. Must be spring on the Surface. It seemed like the whole city had decided to enjoy the sun, even the three brothers that had volunteered to house Sans. For Swap and Blue that was nothing out of the ordinary, but even Stretch had been long gone by the time Sans had woken up. Stretch of all monsters! It boggled the mind, if Sans were the nosy, busybody type he’d be trying to get to the bottom of this.

“Sans. Hey, Sansy.”

Sans looked up to find Red, a fellow skeleton of a more finny variety, darting back and forth just a couple yards away. Sans obligingly sat forward and gave Red his full attention, his eternal grin growing softer around the edges. Red grinned, his false tooth shimmering faint rainbows in the sunlight, then spread every last one of his fins and waved them about.

Like most of the merfolk living down here, Red had far more fins than he used on a daily basis, some so long and delicate looking Sans had no idea what use they’d be. For the most part Red’s fins reflected his rough lifestyle: covered in long healed scars or missing bits altogether. All except for those long, delicate fins, those were just as pristine and brightly colored as anyone else’s in various shades of red with a few streaks of glinting gold. Sans knew because for the whole day Red had been showing them off to Sans, just like now, while looking at him expectantly, just like now. Sans had no idea why.

He watched, unsure how he was supposed to react. Maybe it was some sort of springtime fever? A festival? Some local tradition? Sans had no idea, of all the merfolk flitting about Red was the only one that ever stopped for Sans, but Sans was too confused to ask him about it. After a while of flitting and waving, with Sans sitting there like a lump on log, Red sighed and slunk off. A feat considering he somehow slunk while swimming. With nothing better to do Sans just shrugged off his confusion and went back to lounging. It was the first day since Sans met the sea monsters that he didn’t have fifteen of them all excitedly asking him questions at once, wanting to know all about what it was like living on the Surface with humans. He didn’t mind answering the questions, but it got a little tiring after a while, he was glad for the break.

“Hey, uh, Sansy?” Red was back, only instead of aggressively showing off his fins he was hunched in on himself, nervously hiding something in his hands.

“What’s up, buddy?” Sans sat up and leaned forward again, giving Red his undivided attention.

Red perked up, a few of his finns fanned out as he swam closer. Looking shyer than Sans had ever seen the large, bulky monster look before, he held out his hands. “I uh… I got this. Fer ya.”

Sans leaned a little closer to peer into Red’s outstretched hands. It was a rock. A pretty cool little rock, if Sans were to judge it. About the size of his palm, smooth, with striations in warm, sunset hues. “It’s pretty,” Sans said, still confused.

“Do ya like it?” Red moved a little closer, his hands bobbed in a “here, take it” motion.

“Sure,” Sans said as he took the rock, “ it’s a really nice little rock.”

Red looked incredibly pleased with himself, all his fins flaring around him again. “That’s what you Surface dwellers do, right? Give each other rocks?”

“Uh… not really?” Sans asked with even more confusion. Honestly, if this kept going he was going to become a confusion singularity. He was about to ask more when something moving past them caught his eye. He turned to watch as Stretch swam past, going at a speed he’d never seen the fellow nap enthusiast go before. He headed right over to Red and Edge’s home and suddenly stopped. It wasn’t too far away, but as Sans had learned visibility under water wasn’t really a thing. It was only the nearly neon brightness of Stretch’s yellow, orange, and honey gold fins that let him see them flare and flash as he flitted back and forth much the same way Red had been earlier. It’s not until there’s a flare of bright reds and rusty red-oranges that Sans realizes Edge is sitting outside his home. The pair got close to each other, so close it seems like their fins are getting tangled together. Then they disappear into Edge’s home. And suddenly it all makes sense.

“Sansy?”

Sans looks back up at Red, he had this worried but hopeful expression on his face. “Monsters don’t give each other rocks,” Sans blurts out, he felt his face flush with mana. He looked down, suddenly unable to look Red in the eye socket, “I mean, I’m sure some do, but really any gift is nice. Honestly not a lot of us had space until recently so mostly the only gifts anyone would give would be small or food. But humans sure do like to give each other rocks, all the time. All kinds of rocks: shiny ones, see through ones, colorful ones, glittery ones, some with long dead animals in them. Most are carved or forged into fancy shapes, and some seem to have special meanings. The rocks, not the shapes, though I think some of the shapes have special meanings too? I don’t really get it, honestly…” Sans would have kept babbling but Red was suddenly much, much closer.

“But ya like it?”

“I uh…” Sans swallowed thickly. “Yeah, I really like it.”

Red grinned broadly, mana flushing his own cheeks a soft, rosy color. Every one of his fins flared and waved about happily.

And Sans didn’t have a single fin to respond with, no wonder Red had seemed so confused and disappointed. Sans grabbed him by the ribs and tugged him into a kiss. Red made a few clicking sounds Sans guessed meant he was happy, then opened his mouth to deepen the kiss. Sans was agreeable to that, opening his own mouth and letting their tongues slide over each other, tingling wherever their magics touched. Red pulled Sans closer, pressing their bodies together as the kiss shifted to Red nipping along Sans’s mandible and grazing his sharp teeth down his neck and over his clavicle.

“How about we take this someplace more private?” Sans asked breathily.

“Sounds good, sweetheart.” Red scooped Sans up to carry him bridal style as he swam to Sans’s room. Sans wrapped his arms around Red’s neck and kept him distracted by trailing kisses along the larger monster’s jaw. They were barely in the room when Red pressed Sans down into the soft sand that made up the floor. “Oh, you are a little tease!” He growled before devouring Sans’s mouth in another intense kiss.

Sans took his change to finally feel Red’s thick, sturdy bones. He ran his hands over Red’s broad back, feeling all the little bumps and divots from his active lifestyle, feeling the spark and snap of magic on bone contact when his phalanges slipped between his ribs to graze over the ectoplasm-like flesh there. Red’s hands were doing some exploring of their own, one braced against Sans’s rib cage to hold him steady while the other seemed fascinated with his pelvic girdle.

Red’s body pressed down on Sans, he could feel something long, hot, and heavy press against his pubic symphysis. Sans moaned as the mana that had been gathering steadily around his pelvis solidified into a pussy, already wet for all the good that did under water.

Red stopped cold. He pushed up onto his arms to look down between them, leaving Sans laying there feeling exposed and vaguely embarrassed. “What?” Sans asked, unsure what he was even asking.

“Didn’t know ya could do that.”

“Sure, I got all kinds of tricks up my sleeve.”

Red moved down a little, staring at the bright blue ecto glowing between Sans’s legs. “Never seen what a surface monster looks like before,” he commented absently as a sharp phalange traced around Sans’s lips, but barely grazing along the sensitive false flesh. “Don’t seem that different.”

“We are all monsters,” Sans said with a little hitch to his voice, a little too lost in the sensation of gentle fingers tracing around his entrance. He gasped when one claw grazed over the little knot of leylines that don’t get used in this configuration.

“Let’s see how similar the inside is,” Red said smugly as one finger pushed hesitantly inside, pumping slowly as Red felt his way around.

Sans whimpered and squirmed, spreading his femurs a little wider. His hands raked over the sand, unhappy to find nothing to cling to. “Hnnn… Red!”

“I like it when ya say my name like that, sweetheart.” Red loomed over Sans, grinning down at him. But that put him in range of Sans’s arms, he eagerly reached up to pull the large monster to him. Red let himself be pulled down, finger still slowly working its way a little deeper as he pressed his face to Sans’s sternum. He gently kissed and nipped the bone there, Sans’s breath hitched with every graze of those sharp, shark-like teeth.

“Red,” Sans moaned, “more, please.”

A second finger was added. Sans arched up, pressing his sternum into Red’s kisses. “Heh, ya like that?” He curled his fingers, somehow finding a tangle of ley lines he shouldn’t know about. Then again Sans is pretty sure all monsters have a pretty similar layout for their ley lines. Or normally he’s pretty sure, but with Red knuckle deep inside him he had a hard time stringing complex thoughts together, let alone remember something he learned in high school metabiology.

“S’good,” Sans slurred. “So good, don’t stop.”

“Wasn’t plannin’ on it, sweetheart.” He added a third finger.

Sans gasped and squirmed, “Oh fuck.”

“Mmmm… yeah, gonna fuck ya so good. Gonna stuff ya so full of eggs ya won’t even be able to move.”

“Please,” Sans begged, “please, please, please…”

Red shifted and leaned up and away from Sans, taking his fingers with him. Sans couldn’t help the whimper of frustration, he’d been so close! Red doesn’t leave him that way long, the fingers are quickly replaced with a long, hard shaft rubbing between his lips. The pure magic-on-magic contact sent sparks and a wave of heat up his spine to swirl dizzily in his Soul. Sans gasped and raised his hips, pressing harder into that delicious contact.

“Ya ready, Sansy?” Red all but purred.

“Yes, god yes.”

Red pushed slowly in. Sans expected Red’s dick to be as big as the monster himself, perhaps girthy even. It turned out Red’s dick was actually quite thin, easily slipping in after all that work Red did to loosen him up. And long, really long. He just kept slowly pushing further and further in until it was Sans who bottomed out. Once seated, Red settled down on top of Sans, fully engulfing him in his embrace. At first Sans thought he was giving him a(n unnecessary) moment to adjust, but Red had such a look of concentration on his face that Sans was starting to question that.

There was a bulge at the base of Red’s dick. A knot? Was he cumming already? But the bulge was moving, seemingly traveling up Red’s dick. There was the burn Sans expected as he was spread, the bulge wide enough to press against his walls as it moved. The bulge started to squeeze past his G-spot and Sans keened, seeing stars. Just like that his body was ramped back up and teetering on the edge. Then the bulge reached the end and started to press and press and press into him, the pressure building more and more until something finally gave and it seemed to pop into place. Sans screamed, the sudden release of pressure tipping him over the edge so hard his spine arched until he thought it would snap in two.

When Sans scraped up enough brain power and breath to speak he croaked out, “Holy shit.” By that point there’s already another bulge on it’s merry way.

“You okay, Sans? Should I stop?”

“Don’t you dare!” Sans wrapped his legs around the base of Red’s tail and squeezed, trapping the large monster.

Red just laughed and went back to comfortably relaxing on top of Sans. Another bulge starts in, just as the second one gets to the end. Sans gasped and cursed, the build up of then sudden release of pressure a sweet relief that sent a pulse of heat and pleasure through his ley lines. The build up towards that precipice was slower this time as more bulges keep coming, one right after the other.

“Oh fuck me.” He’s getting his insides massaged!

“That’s what I’m doing, sweetheart.”

Sans laughed, though it was interrupted by a gasp as a particularly large bulge lit up his insides. Sans choked on the gasp and clung Red as he tipped over the edge.

“That good for ya?”

Sans didn’t even know how to articulate how good it felt, so he just pulled Red into a sloppy, fuck drunk kiss. Red seemed quite happy to reciprocate if the way he set about devouring his mouth was anything to go by.

Sans reluctantly broke the kiss with a grunt, there was some kind of uncomfortable pressure on his middle. That was when Sans realized he currently had a middle. At some point the amount of ecto he was wearing had expanded to span from his knees up to his ribs, giving him a pair of thighs and a tummy that had bloated to press uncomfortably into Red’s muscular abs.

Red leaned up and away, the change in angle making Sans’s toes curl, to look down at Sans’s middle with a soft, gentle smile, one hand coming up to caress the side of his now bloated belly. That’s when the realization belatedly crashes through his pleasured haze: eggs. Red was being very literal when he had talked about stuffing Sans full of eggs.

Sans reached for Red, “Wanna touch you.”

“Whatever ya want, sweetheart.” Red had to be careful of Sans’s still growing belly, but he went back to looming over Sans, bracing his arms on either side of him. Sans clung to those arms to ground himself as he reached his peak again, riding out a wave of pleasure that washed over him. By the time he washes ashore he finds himself left adrift with tears gathering at his socket corners and more already drifting in the water near his face.

“Sansy? Ya still with me?”

At first Sans nods, then changes his mind and shakes his head.

“Alright, I’m gonna pull out now.” Red did just as he said, slowly pulling away from Sans.

Sans whimpered and would have protested, but he found his legs were as limp as soggy spaghetti noodles and wouldn’t cling to Red. Once he’s out Red tensed and shuddered, then collapsed on the sand next to Sans, one arm thrown haphazardly over his ribs.

Sans tried to nudge Red, but ended up kind of smacking him. Gently. Mostly because he didn’t have any strength left in his bones. “Red, gonna float ‘way.”

Red propped himself up on an elbow, “Ya feel that spacey?”

Sans shook his head, “Buoyant.” He’s not sure where he found the second brain cell to rub with his last one to find that word, but he’s sure glad he did.

Red curiously lifted his arm, Sans lifted with it. “Oh shit!” He pulled Sans back down into his arms, tucking him against his body so Sans was the little spoon and practically draped over him. “There, how’s that?”

Sans felt much more secure like that, enough to yawn and relax into Red’s hold. “Better.”

“Good, get some rest. I’ll get ya something to eat later.”

Sans just sunk further into Red’s hold, ready to sleep for a week after all that. Just as he started to drift off, one of Red’s hands moved to caress over his swollen belly again.

“You’re real proud of yourself, huh?” Sans asked sleepily.

“Fuck yeah I am, can’t wait to show ya off to Boss.”

Sans groaned and rolled over so he could hide his face in Red’s chest. He would have to pull out his phone and send off a ton of messages, he was sure the darn thing would be blowing up after that. But it could wait until after his nap. He settled down and let Red’s hand running up and down his spine sooth him into a well deserved nap.

**Author's Note:**

> There was no where to put it in the fic but he has one of Alphys's super awesome magi-tech phones that works fine under water up to some pretty deep pressure. The audio on it is meh because under water, but the camera and keyboard work just fine so Sans usually just texts anyone he wants to chat with. The price for that is feeding Alphys all the material for her friendfictions.


End file.
